The S Stock is a fleet of subsurface rolling stock utilized on the London Underground. Delivered by Bombardier Transportation in Derby, the S Stock was ordered to replace the A60, A62, C69, C77 and D78 stock on the Metropolitan, District, Hammersmith & City, and Circle lines. The order consists of a standardised fleet of 191 trains (1,395 cars). The stock has two types, S7 and S8, serving different lines, with differences in the arrangement of seating and number of cars. Both types have air-conditioning and low floors to ease accessibility for disabled people, and it is also possible for a passenger to walk from one car to the next while the train is moving, unlike older subsurface trains.

The order is said to be the biggest single rolling-stock order in Britain.[1] According to Transport for London, the cost will be £1.5 billion.[2]

Passenger service began on the Metropolitan line in July 2010, the Hammersmith & City line in July 2012, and the Circle and District lines in September 2013. The S Stock completely replaced the A Stock on the Metropolitan line in September 2012, and the C Stock on the Circle and Hammersmith & City lines in February 2014, and on the District line in June 2014; and is expected to fully replace the D Stock on the rest of the District line in 2016.

Part of Bombardier's Movia family, the stock consists of 133 S7 seven-car trains for the Circle, District and Hammersmith & City lines and 58 S8 eight-car trains for the Metropolitan line. S8 Stock entered service between 2010 and 2012, operating all services by September 2012.

The stock has faster acceleration than previous stock, at 1.3 m/s2, but its top speed is 62 mph (100 km/h), 8 mph slower than A Stock but faster than C and D Stocks. During the period of dual operation with both old and new trains, the stock has its performance capped to match that of the older trains in order to comply with signalling constraints and avoid bunching of the service. S8 stock seats 306 passengers compared with 448 for A Stock, a reduction of 32%, but it can accommodate 25% more standing passengers (1,226 compared with 976) and has dedicated space for wheelchairs.[1]

Eventually the voltage will be increased from nominal 630 volts to 750 volts. This will allow for better performance and also for the increased power demands of air-conditioned, fully-motored-axle trains, and allow the trains to return energy to the network through regenerative braking.[6]

At present the stock is operated manually, but London Underground will activate automatic train operation once the necessary signalling is in place by 2018.[6]

The stock is air-conditioned throughout: the sub-surface tunnels (unlike deep-level tube lines) allow the exhausted hot air to disperse,[1] and two-thirds of the sub-surface network is in the open air.[7] The stock has regenerative brakes, returning around 20% of their energy to the network and thus increasing energy efficiency.[8]

End external displays show two lines of text: the top line for the destination, and the bottom for the line. Internally, it has larger dot matrix indicators (DMIs) than D Stock (C Stock lacks DMIs, as did A Stock). The DMIs show destination and line, and can display other messages such as safety notices. There are also DMIs on the exterior, with text alternating between destination and line, and on S8 stock the type of service, i.e. fast, semi-fast or all stations.

Interior of an S8, with passenger information display and some transverse seating

Another new feature is that to prevent accidental pressing of emergency alarms there are flaps over the alarm buttons where the wheelchair spaces are.

The air-conditioning system, the first on London Underground trains, is by Mitsubishi and has two circuits so that if one fails there is still 50% capacity.[7] Through gangways from car to car are another innovation, as on the London Overground's Class 378. This allows passengers to move from crowded cars to ones with more room, provides extra room for standing and creates a sense of security. CCTV enables the driver to see into every car, while track-to-train video links give the driver a view of the train exterior before leaving a station.[7]

There is a fold-out set of steps in each driver's cab to allow fast evacuation in an emergency.[9]

Interior of a special needs car of an S8 Stock train with folding seats and space for 'personal wheeled transports' such as wheelchairs and children in a pushchair

S8 Stock was initially tested overnight between Amersham and Watford via the Watford North Curve from 9 November 2009. Driver training began in early January 2010, and the first train entered revenue service on 31 July 2010, shuttling between Wembley Park and Watford.[7]

By 27 June 2011, S8 Stock was running along the whole Metropolitan line. Deliveries were suspended by Transport for London in November 2011 due to concerns over reliability. A number of train were delivered to London Underground's Neasden depot, but were not accepted to enter service. Deliveries resumed in mid-December 2011.[12]

In August 2012, London Underground confirmed that strap handles would be introduced on S8 Stock, in response to passenger complaints over the height of the handrails as compared with A Stock.[13]

By 15 September 2012, all 58 S8 trains had been delivered to Neasden Depot. The elderly A Stock trains were completely withdrawn 11 days later. In November 2012 it was reported that 37 of the new trains would be sent back to Bombardier for urgent modification at Bombardier's cost, and that drivers were unhappy with the driver's cabs.[14]

The first trains entered service on 6 July 2012, from Hammersmith to Moorgate. S7 trains are longer than those they are replacing (seven cars and 117 m long instead of six cars and 93 m long), so some station platforms are having to be lengthened before they can operate in normal service. For stations where this has proved physically impossible, such as Baker Street, the trains have a selective-door-opening capability whereby the doors at the end of the train will not open.[7]

On 4 December 2012, an S7 train went east of Moorgate to Barking for testing. This was the first time an S7 train had been seen in peak hours. S7 Stock began operating a full service from Hammersmith to Barking on 9 December 2012.[15] The line was completely operated with S7 stock by 10 February 2014.[citation needed]

The District line was the last line to receive the S Stock due to most of its trains being newer and having been more recently extensively refurbished. The first S7 Stock train on the District line entered passenger service between Olympia and West Ham on 2 September 2013.

On 6 February 2014, the S Stock started running between Wimbledon and Edgware Road. On 13 June 2014, the S Stock started services to Ealing Broadway and on 17 June saw the start of commercial service to Richmond. On 16 January 2015, the service began running to Upminster.

Since the D Stock was extensively refurbished in 2005-08 and the older C Stock has been described by London Underground as being "in an increasingly poor state", the C Stock was withdrawn first, with the C Stock replaced in 2014 and the D Stock to be replaced at the end of 2016.[17][18]